I am a bit puzzled by the way to handle server upgrade planning for software we maintain on remote sites. This is my case:

I work for a software company that has many external clients. We are trying to be more Agile in our development so we plan to release small improvements every quarter and we wish to keep our clients informed of maintenance schedules. Instead of having angry clients that believe there ROI of our support plan is low, we want to be more proactive.

Lets say we have 100 machines to take care of, is there some tool to assist me in planing the maintenance with clients? Right now I get a call from a client that is unhappy requesting we upgrade them, that is when we go into panic mode and start making calls. That is when I need to check my calendar, coordinate with the other guys, call a few times, change the date again and again until everyone is happy. Can this be done better?